Solid polymer electrolyte fuel cell power plants are known in the prior art, and prototypes are even available from commercial sources, such as Ballard Power Systems, Inc. of Vancouver, Canada. These systems are serviceable, but are relatively complex. An example of a Ballard Power Systems polymer membrane power plant is shown in U.S. Pat. No. 5,360,679, granted Nov. 1, 1994.
In addition, known fuel cell constructions commonly include a proton exchange membrane disposed between respective cathode and anode plates. In general, the operation of a proton exchange membrane (PEM) fuel cell includes the supply of gaseous fuel and an oxidizing gas to the anode electrode plate and cathode electrode plate, respectively, and distributed as uniformly as possible over the active surfaces of the respective electrode plates, or, more specifically, the electrode plate surfaces facing the proton exchange membrane, each of which typically includes a catalyst layer therebetween. An electrochemical reaction takes place at and between the anode plate and cathode plate, with attendant formation of a product of the reaction between the fuel and oxygen, release of thermal energy, creation of an electrical potential difference between the electrode plates, and travel of electric charge carriers between the electrode plates, with the thus generated electric power usually constituting the useful output of the fuel cell.
One problem occurring in solid polymer fuel cells relates to the management of water, both coolant and product water, within the cells in the power plant. In a solid polymer membrane fuel cell power plant, product water is formed by an electrochemical reaction on the cathode side of the cells, specifically by the combination of hydrogen ions, electrons and oxygen molecules. The product water must be drawn away from the cathode side of the cells, and makeup water must be provided to the anode side of the cells in amounts which will prevent dryout of the proton exchange membrane, while avoiding flooding of the cathode.
Austrian Patent No. 389,020 describes a hydrogen ion-exchange membrane fuel cell stack which utilizes a fine pore water coolant plate assemblage to provide a passive coolant and water management control. The Austrian system utilizes a water-saturated fine pore plate assemblage between the cathode side of one cell and the anode side of the adjacent cell to both cool the cells and to prevent reactant crossover between adjacent cells. The fine pore plate assemblage is also used to move product water away from the cathode side of the ion-exchange membrane and into the coolant water stream; and to move coolant water toward the anode side of the ion-exchange membrane to prevent anode dryout. The preferred directional movement of the product and coolant water is accomplished by forming the water coolant plate assemblage in two parts, one part having a pore size which will ensure that product water formed on the cathode side will be wicked into the fine pore plate and moved by capillarity toward the water coolant passage network which is inside of the coolant plate assemblage. The coolant plate assemblage also includes a second plate which has a finer pore structure than the first plate, and which is operable to wick water out of the water coolant passages and move that water toward the anode by capillarity. The fine pore and finer pore plates in each assemblage are grooved to form the coolant passage network, and are disposed in face-to-face alignment between adjacent cells. The finer pore plate is thinner than the fine pore plate so as to position the water coolant passages in closer proximity with the anodes than with the cathodes. The aforesaid solution to water management and cell cooling in ion-exchange membrane fuel cell power plants is difficult to achieve due to the quality control requirements of the fine and finer pore plates, and is also expensive because the plate components are not uniformly produced.
In the fuel cell technology, the water transport plate is a porous structure filled with water. During fuel cell operation, the water transport plate supplies water locally to maintain humidification of a proton exchange membrane (PEM), removes product water formed at the cathode, removes by-product heat via a circulating coolant water stream, conducts electricity from cell to cell, provides a gas separator between adjacent cells and provides passages for conducting the reactants through the cell. The water transport plate supplies water to the fuel cell to replenish water which has been lost by evaporation therefrom. This system and operation thereof is described in U.S. Pat. No. 5,303,944 by Meyer, U.S. Pat. No. 5,700,595 by Reiser and U.S. Pat. No. 4,769,297 by Reiser, each incorporated herein by reference.
In addition, the environmental and operational parameters of a water transport plate must be carefully balanced to obtain optimum performance of the overall fuel cell. For example, parameters of the water transport plate such as pore size, resistivity, particle size, resin content, yield strength and coolant flow rate therein must be properly selected to obtain bubble pressure characteristics and water permeability which are acceptable for efficient operation of the fuel cell.
A major concern with PEM fuel cells is the water management with the cell. This is of particular concern when employing porous members such as the water transport plates discussed above. This porosity is needed to supply to and substantially uniformly distribute over the respective active surface the respective gaseous medium which is fed through respective channels provided in the anode water transport plate and the cathode water transport plate to the areas of the respective electrode plate that are spaced from the proton exchange membrane. Also, these porous structures are used to remove the reaction water from one of the active surfaces and to supply water to the other active surfaces to avoid drying out of the proton exchange membrane.
There is a particular concern in PEM fuel cells relating to reactant cross-over into the coolant channels. As can be understood such cross-over greatly diminishes the efficiency of the cell and can lead to a hazardous condition. In a water transport plate, a reactant gas barrier and water permeable member can only be formed and maintained when the pores in the water transport plate are filled with water. If the water transport plate of the PEM cell ceases to be filled with water, local dryout of the cell can easily occur resulting in the aforementioned problems. When he fine pores of the water transport plate dry out, reactant gases an easily break through, even at low pressures.
There are several causes that result in gas bubbles within the coolant. Bubbles can form due to residual gases within the coolant channels during start-up, outgassing of dissolved gases in the coolant due to changes in the coolant temperature, and due to mechanical leakage since the coolant system operates below ambient pressure. These bubbles may locally accumulate in the coolant flow channels resulting in a section of the cooler that is not supplied with the coolant. As the size of the bubble increases, in-plane permeability of water within the water transport plates is inadequate to maintain saturation of the water transport plate, and local dryout of the water transport plate occurs. This results in reactant leakage into the coolant system, and local over-heating of the cell, both of which creates a dangerous condition. In known PEM cells, the difficulty in effectively flushing out the gas bubbles exacerbates the problem of local dryout.
Typically, PEM fuel cells have water transport plates with vertically positioned coolant channels with upward and downward flowing legs. Gas bubbles can get trapped in the downward flowing leg of the coolant channel configuration. Once enough adjacent channels are filled with gas, the performance of the water transport plate is greatly hindered. With a portion of the water transport plate deprived of coolant water, the water transport plate must rely solely on its in-plane permeability to supply the reactant gases with water for saturation. However, at high power levels which are equivalent to increased cell temperatures, the in-plane permeability of the water transport plate is not enough to keep up with the water demand within the cell. As a result, the water transport plate begins to dry out.
Dryout of the water transport plate diminishes its ability to effectively act as a gas barrier. At a low enough water fill level, the plate allows for reactant gas to cross over into the coolant stream. At this point, the water transport plate does not have the ability to recover unless the trapped gas bubbles are physically swept out of the coolant flow field to permit water to flow over the dry areas to rewet the water transport plate.
In view of the foregoing, an improved fuel cell is desired which includes the ability to prevent local dryout of the water transport plate of the PEM fuel cell for efficient fuel cell operation. It is also desirable for a PEM fuel cell to include the ability to flush trapped reactant gas bubbles from the coolant flow field.